At present, Device to Device (D2D) communication technologies have already been accepted by the third Generation Partnership Project (3GPP) standard, depending on great potential value thereof in public safety field and general civil communication field. Standardizations of some functions of D2D communication technologies have already been achieved in 3GPP Rel-12, which include mutual discoveries of D2D terminals in a cell coverage secarino, and broadcast communications among D2D terminals in a In Coverage (IC) scenario, a Partial Coverage (PC) scenario and an Out of Coverage (OC) scenario.
At present, 3GPP Rel-12 standard defines two kinds of D2D broadcast communication modes, which are referred to as Mode 1 and Mode 2 for short. Mode 1 requires that a user equipment (UE), which transmits a D2D broadcast communication, must be an in coverage UE (ICUE) under a cellular network. The UE obtains configuration information of a physical sidelink control channel (PSCCH) resource pool of Mode 1, by receiving system broadcast signaling from an evolved Node B (eNB). Foregoing configuration information of PSCCH resource pool includes a PSCCH period, a position of a subframe for transmitting PSCCH within each period, and a position of a physical resource block (PRB) for transmitting PSCCH within each subframe. When there is data for a UE supporting Mode 1 broadcast communication, the UE applies for dedicated Mode 1 communication resources to an eNB via a specific buffer status report (BSR). Subsequently, the UE senses sidelink grant of the eNB before each PSCCH period, obtains a position of a resource for transmitting PSCCH and physical sidelink shared channel (PSSCH) within the PSCCH period. In Mode 1, resource conflicts among different UEs may be avoided, through centralized control of the eNB.
A UE transmitting D2D broadcast communications in Mode 2 may be an ICUE, or may be an OCUE outside the coverage of a cellular network. The ICUE obtains a PSCCH resource pool of Mode 2, and configurations of an associated PSSCH resource pool, by receiving broadcast signaling of an eNB system. The PSSCH resource pool includes a position of a subframe for transmitting PSSCH within an associated PSCCH period, and a position of a PRB for transmitting PSSCH within each subframe. Randomly select resources for transmitting PSCCH and associated PSSCH within each PSCCH period. The OCUE determines a PSCCH resource pool of Mode 2, and configurations of an associated PSSCH resource pool, by using preconfigured information. Resource selection mode of the OCUE is the same as that of the ICUE. In the PC scene, configurations of a Mode 2 resource pool preconfigured by an OCUE are relevant with carrier frequency, system bandwidth, and/or, time division duplexing (TDD) configurations of a cell located by an ICUE, which participates in a D2D broadcast communication.
In foregoing two kinds of D2D broadcast communication modes, the PSCCH resource pool and the PSSCH resource pool, or PSSCH resource are bound with a one-to-one correspondence. Within each PSCCH period, the PSCCH resource pool is located before the associated PSSCH resource pool, or PSSCH resource, and resources thereof are not overlapped. In addition, a D2D terminal always operates in a half-duplex mode. Thus, terminals simultaneously transmitting cannot receive a signal from a peer end. In Rel-12, each PSCCH will be transmitted twice within each PSCCH period. Each PSCCH transmission occupies one PRB. And foregoing half-duplex limitations may be solved, by using resource hopping. For example, for PSCCH transmitted with the same subframe in a first transmission, an offset may be generated for a position of a subframe transmitting such resource in a second transmission. Offset magnitude is relevant with frequency domain position of the first resource transmission. Thus, for PSCCH transmitted with the same subframe in the first transmission, it is guaranteed that position of subframe for re-transmitting the PSCCH is different. Besides, two transmissions may ensure reliability of PSCCH reception.
FIG. 1 illustrates a structure of an uplink subframe in a 3GPP D2D system. In 14 Orthogonal Frequency Division Multiplexing (OFDM) symbols of one subframe, two OFDM symbols are used for transmitting a Demodulation Reference Signal (DMRS). Symbol indexes of the foregoing two OFDM symbols are respectively 3 and 10. The last OFDM symbol of the subframe is punctured fixedly, which is used for generating transceiver conversion time of a device, and avoiding overlapping of two adjacent subframes resulted from problems, such as transmission delay, timing advance. Other symbols are used for transmitting uplink data. Here, the first OFDM symbol of the subframe also transmits data. However, when implementing practically, this OFDM symbol may be taken as automatic gain control (AGC).
Since 3GPP D2D communications mainly aim at low speed terminals, and V2X services with lower requirements of delay sensitivity and reception reliability, D2D functions already achieved are far from meeting users' requirements. In each subsequent 3GPP version, it is a wide consensus to further enhance the D2D functional framework, which is achieved by each communication terminal manufacture and communication network equipment manufacture at present. Based on current D2D broadcast communication mechanism, Vehicle to Vehicle/Pedestrian/Infrastructure/Network (V2X), which supports a direct low-delay communication with higher-reliability between high-speed devices, between a high-speed device and a low-speed device, between a high-speed device and a stationary device, is one function needing to be standardized firstly.
Requirements of main application scenes of D2D are met by the structure of the uplink subframe, which is shown in FIG. 1. However, for a typical V2X application scene, e.g., the highest UE relative speed supported by the V2X communication is 500 km/h, the carrier frequency may achieve 6 GHz, serious inter-subcarrier interferences may be brought about by Doppler shift resulted from high-speed and high carrier frequency. In addition, take into account influences of timing offset and frequency offset between a base station and a UE, performance requirements cannot be met by the DMRS structure shown in FIG. 1. In current discussions of standardization conferences, FIG. 2 shows an important solution, that is, DMRS is transmitted on 4 OFDM symbols, so as to improve time density of DMRS. In other words, indexes of DMRS symbols are respectively 2, 5, 8 and 11, so as to provide better performances.
In a 3GPP D2D system, DMRS sequences of PSCCH are fixed. That is, all the transmitters adopt the same DMRS sequence. Specifically, on the basis of a DMRS generation method in a long term evolution (LTE), the root sequence of the DMRS is obtained based on a physical cell identity (PCID) 510. Cyclic shift (CS) of the DMRS is fixed to be 0. Orthogonal cover code (OCC) of the DMRS is fixed to be [1 1]. Scrambling sequence of scheduling information carried by PSCCH is also fixed. That is, all the transmitters adopt the same scrambling sequence. Specifically, on the basis of a scrambling-code generation method in the LTE, PCID is configured to be 510. Other information, such as a time slot index, a UE ID, is fixed to be 0. Based on such method, when two devices transmit SA on the same PRB, DMRSs of the two devices are completely overlapped, which is equivalent to one DMRS sequence at a receiver. Since terminal density in the V2X communication is much higher than that in D2D, the probability for at least two devices to transmit SA and/or data on the same resource is greatly increased, that is, a case where SA resources are conflicted. In addition to foregoing conflicts, even if two sending devices transmit data with different frequencies of the same subframe, reception performance may also be reduced by in-band leakage interference, taking into account influences of distance effect. That is, for a receiver, compared with energies given away to other adjacent PRBs by a device very close, energies on these adjacent PRBs coming from remote devices may be in the same order of magnitude, or even weaker. Since terminal density in the V2X communication is much higher than that in D2D, foregoing in-band leakage interference may become even worse.
Based on discussions in current standardization conferences, one solution is to solve foregoing problems of collision and in-band leakage based on sensing. One basic assumption here is to occupy resources by a device with semi-persistent scheduling (SPS). That is, resources occupied by a device are periodical within a duration. As shown in FIG. 3, a time point that a device selects PSCCH/PSSCH resources is represented with subframe n. Within a time period from subframe (n−a) to subframe (n−b), a device firstly senses resources within a resource pool thereof, and determines which time-frequency resources are occupied and which time-frequency resources are idle. And then, the device selects PSCCH/PSSCH resources within subframe n. Denote that PSCCH is transmitted within subframe (n+c), PSSCH is transmitted within subframe (n+d), and a reserved resource is in subframe (n+e). Subsequently, the device transmits PSCCH in subframe (n+c), transmits PSSCH in subframe (n+d), and transmits a next data within the reserved resource of subframe (n+e). More particularly, when c is equal to d, PSCCH and PSSCH are located within the same subframe. There are two modes for foregoing device to sense resources within the resource pool thereof. A first mode is to obtain accurate information of channels occupied by other devices, after decoding PSCCH, so as to measure received power of a corresponding device. The other mode depends on sensing energy of a PSSCH resource pool. The first method may obtain accurate information about channel occupation and reservation. However, when the PSCCH is not correctly received, e.g., when PSCCH collisions of multiple devices occur, the PSCCH-based sensing is failed. The second method is to determine whether resources are occupied, based on level of sensed energy, so as to avoid using occupied resources as much as possible. However, since a V2X service is not strictly periodic, message periods of various devices may also be different within a time period, which affects predictive performances of the energy-based sensing method. Practically, the PSCCH-based sensing and energy-based sensing may be used in combination, so as to avoid collision and interference as much as possible, and improve performances. When the PSCCH is correctly received, how to sense energies of the PSCCH is an urgent problem to be solved.